Various systems for making plate directly from digital data from computer have heretofore been developed. For example, a photopolymerization system image-forming material adapted for exposure to blue or green visible light from laser has been noted as a photosensitive layer for lithographic printing plate which can attain a high press life by the toughness of coat layer cured by photopolymerization in that it is sensitive to argon laser beam and a direct plate-making of high sensitivity utilizing a photopolymerization initiation system is made possible.
By way of examples, as a laser printing plate utilizing a photopolymerization initiation system sensitive to visible laser beam such as argon laser beam there has been used one comprising a photopolymerizable composition layer comprising a compound having an addition-polymerizable ethylenically double bond, a photopolymerization initiator and optionally an organic high molecular binder and a heat polymerization inhibitor provided on an aluminum sheet as a support, and a barrier layer to oxygen that inhibits polymerization provided thereon. Such a photopolymerizable lithographic printing plate is exposed to a desired image so that the exposed area undergoes curing by polymerization, and then subjected to removal of the unexposed area with an alkali aqueous solution (development) to form an image thereon.
In recent years, laser has been remarkably developed. In particular, high output and small-sized solid lasers and semiconductor lasers emitting infrared rays having a wavelength of from 760 nm to 1,200 nm have been easily available. These lasers are very useful as a recording light source for use in direct plate making from digital data from computer, etc. To this end, a material capable of recording from these infrared laser beams has been desired in addition to the aforementioned practically useful photosensitive recording materials sensitive to visible light range of not higher than 760 nm. For example, a negative-working image-recording material having a photosensitive layer comprising a photopolymerizable composition containing an infrared absorber, a photo or heat polymerization initiator and a compound having a polymerizable or crosslinkable functional group is known as described in Japanese Patent Laid-Open No. 2000-187322.
A photopolymerizable composition, regardless of being cured by visible light or infrared rays, essentially comprises a polymerization initiator such as radical generator, a monomer having a polymerizable functional group and a binder polymer for enhancing the film properties of photosensitive layer. When the photopolymerizable composition is exposed to light or heated, the initiator in the exposed (heated) area generates a radical which causes the polymerizable compound to undergo polymerization/crosslinking reaction causing the curing reaction of the photosensitive layer resulting in the formation of image area. Such a photopolymerizable composition contains a large amount of a polymerizable compound which is mostly a compound having a relatively low molecular weight such as monomer and oligomer.
Lithographic printing plate precursors are normally packaged under light shielding and then transported or stored in various environments.
A negative-working photosensitive layer comprising a photopolymerizable composition which cures upon exposure to light is disadvantageous in that when stored at high temperature over an extended period of time, polymerization of polymerizable compound occurs in the dark to insolubilize unexposed area, resulting in image defect such as poor development. Particularly, in an image-forming material which is exposed to infrared laser and then directly developed without heat treatment, it is difficult to fulfill both sensitivity and preservation stability.